溢流阀是安装在液压泵的出口处,它的作用是稳压、安全卸荷和背压。
参考答案：对
为适应提速要求，一段铁路需要更换铁轨，用每根12米长的新铁轨，换下每根长为9米的旧铁轨．这段铁路原来有168根铁轨，需换成多少根新铁轨？
______ at the terrible news that I didn’t know what to say.
A．So sad Mary looked
B． So sad did Mary look
C．So sadly Mary looked
D． So sadly did Mary look
